An absorbent core containing a hydrophilic fiber such as pulp and a water-absorbent resin as its components is widely used in sanitary materials such as disposable diapers, sanitary napkins, incontinence pads and the like, in order to absorb body fluids.
Recently, the sanitary material has higher performance and a thinner size, and an amount of the water-absorbent resin used for each sanitary material tends to increase, and also a ratio of the water-absorbent resin tends to increase with respect to the whole absorbent core constituted of the water-absorbent resin and a hydrophilic fiber. That is, by using (i) a smaller amount of a hydrophilic fiber whose bulk density is low and (ii) a larger amount of a water-absorbent resin having a superior water absorbing property and high bulk density, a ratio of the water-absorbent resin contained in the absorbent core is increased, thereby making the sanitary material thinner without decreasing an amount of water absorption.
However, the sanitary material which includes a smaller amount of the hydrophilic fiber and a larger amount of the water absorbent resin is preferable merely in terms of liquid storage, but raises problems in terms of distribution and diffusion of liquid in actual use in diapers. For example, when a large amount of the water absorbent resin is used, the water absorbent resin becomes soft and gelatinous upon absorbing water. This causes a gel blocking phenomenon. As a result, a liquid diffusing property of the diaper significantly drops. In order to avoid such phenomenon and to keep high absorbing property of the absorbent core, a ratio of the hydrophilic fiber and the water absorbent resin is inevitably limited, so that there is a limit in making the sanitary material thinner.
In view of this, for example, Patent document 1 discloses that an internal cross-linking agent for forming cross-linkage by covalent bonding and an internal cross-linking agent for forming cross-linkage by multivalent metal bonding are used in combination as an internal cross-linking agent for a water-absorbent resin so that a water-absorbent resin having a low water absorption rate is produced. In Patent document 1, multivalent metal exists evenly inside the water-absorbent resin. Also, Patent document 1 discloses that a produced water-absorbent resin is subjected to surface cross-linking with an organic cross-linking agent.
For example, Patent documents 2 through 4 disclose that for production of a water-absorbent resin, surface treatment by multivalent metal and organic surface cross-linking treatment with a surface cross-linking agent are used in combination. The combination use mainly exerts the following effects: prevention of flocculation of a water-absorbent resin and enhancement of liquid permeability. In this case, multivalent metal exists only in the vicinity of the surface of the water-absorbent resin particle.
Patent document 5 discloses a process for producing a water-absorbent resin by agglomeration. Patent document 5 also discloses that multivalent metal is used as a surface cross-linking agent.
However, even when such a water-absorbent resin is used in an absorbent core, the water-absorbent resin is not enough to maintain absorption properties of the absorbent core.
By the way, a water-absorbent resin is a cross-linked polymer having water-absorbing property. A vicinity of the surface of the water-absorbent resin particle can be subjected to surface cross-linking treatment for forming a cross-linked structure therein. The surface cross-linking treatment is performed to appropriately control water-absorption properties of the water-absorbent resin particles so that various performances are balanced.
Generally, it is preferable that a water-absorbent resin has a low content of fine powder (water-absorbent resin fine particles). If a water-absorbent resin used in sanitary material has a high content of the fine powder, the fine powder forms a gel block in the sanitary material when the sanitary material absorbs urine. Formation of the gel block becomes a factor of lowering liquid permeability.
In the case of handing a water-absorbent resin having the fine powder remaining therein, the fine powder dissipates in the form of powder dust. This becomes a factor of lowing yield of a water-absorbent resin. Further, the fine powder in powder dust form may have industrial safety and health concerns. Here, the fine powder is, for example, a fine powder composed of fine particles each having a particle diameter of not more than 150 μm, and is generated in the process of the production for a water-absorbent resin. Especially, a powder dust which is considered to have industrial safety and health concerns is, for example, a fine particle having a particle diameter of not more than 10 μm. Examples of a form that the fine powder takes include a fine particle gel during reversed suspension polymerization and a fine powder that occurs in the process of pulverizing a dried polymer gel used for production of a water-absorbent resin, or in the process of classifying obtained pulverized particles.
One example of a method for solving the problem is to collect the fine powder in the process of the production for a water-absorbent resin so that the collected fine powder is reused (recycled) in the previous step of the production process for a water-absorbent resin. However, the fine powder is agglomerated for its reuse because the fine powder itself is difficult to handle. Some methods for agglomerating the fine powder have been suggested (e.g. Patent document 6).
The present inventors have been aware of the following problem associated with agglomeration of fine powder.
That is, the agglomeration is performed by adding a water base liquid to a fine power. However, the fine powder has a large surface area. Because of this, in the case of a fine powder which is not subjected to surface cross-linking treatment, fine particles at portions of the fine powder where they are in contact with the water base liquid quickly absorb the water base liquid. As a result of this, fish eye (nonuniform agglomeration) occurs, and uneven gelation thus occurs. This decreases the efficiency in agglomeration and increases a rate of returning from an agglomerated product to a fine powder.
On the other hand, in the case of a fine powder having been subjected to surface cross-linking treatment, nonuniform agglomeration is less likely to occur. However, the fine powder having been subjected to surface cross-linking treatment has a low surface bonding property due to the surface cross-linking treatment, and fine powders are therefore weakly bound together. Thus, the agglomerated product is easy to damage under a mechanical impact. The production process for a water-absorbent resin includes the process of pulverizing a dried polymer gel and the process of classifying obtained pulverized particles. Thus, upon mechanical impact during these processes, the agglomerated product can possibly turn back to fine powder form. Even after the agglomerated product is turn into commercial products, the agglomerated product can possibly be damaged during storage of the commercial product and in the process of using the agglomerated product in sanitary materials or other applications.
[Patent document 1]
U.S. Patent Application Publication No. 2002/193492
[Patent document 2]
U.S. Pat. No. 632,352
[Patent document 3]
Japanese PCT National Phase Unexamined Patent Publication No. 539281/2002
(Tokuhyo 2002-539281)
[Patent document 4]
International Publication WO04/113452
[Patent document 5]
Japanese Unexamined Patent Publication No. 254429/1999
(Tokukaihei 11-254429)
[Patent document 6]
Japanese Unexamined Patent Publication No. 106514/1999
(Tokukaihei 11-106514)
[Patent document 7]
Japanese Unexamined Patent Publication No. 254429/1999
(Tokukaihei 11-254429)